This invention relates to improved polyester molding compositions characterized by being tack free and by little or no tendency toward shrinkage during the curing thereof, and which when cured produce molded articles having superior surface smoothness; that is, "low profile", or little variation in surface smoothness. More particularly, it relates to polymerizable polyester compositions comprising a compatible mixture of (1) an unsaturated polyester and an ethylenically unsaturated monomer copolymerizable therewith, and (2) a styrene-maleic anhydride copolymer reacted with a hydroxyl or amine terminated saturated polyester.
It has long been known to prepare linear polyesters by the reaction of dihydric alcohols and ethylenically unsaturated dicarboxylic acids, and that such unsaturated polyesters are capable of addition polymerization with unsaturated monomers to form thermoset polymers. It is also known to admix such linear ethylenically unsaturated monomers, such as styrene, and to heat the resulting mixture, in the presence of a catalyst, such as a free radical generating substance, to effect copolymerization.
It has been suggested to add to the above mixture of unsaturated polyester and copolymerizable monomer, an unpolymerizable or saturated linear polyester, which mixture can then be polymerized or cured to a substantially infusible state to produce products of improved strength. Such polyester compositions are often reinforced with fillers, such as fiber glass, either in the form of rovings or mats, and the molded products prepared therefrom have found extensive application in industry, for example in floor panels for or portions of automotive bodies, for appliance shells, boat hulls, and the like. However, in the formation of shaped articles with such reinforced plastic composition, difficulties arise because the fibers initially immersed within the uncured liquid resin tend to protrude from the surface causing the object, after molding and curing of the resin, to have irregularities on the surface. Moreover, when the compositions gel to form thermoset cured compositions shrinkage may occur, and the cured piece may develop very small cracks or "microcracks". Such shrinkage, microcracks, and surface irregularities are particularly undesirable where a molded article having a very smooth surface is required. Shrinkage results in the separation of the minute areas of the surface of the cured product from the mold surface, with the result that the surface exhibits poor gloss, minute irregularities or unevenness and is not of the desired smoothness. The molded article fails to accurately reproduce the dimensions and surface characteristics of the mold and results in either rejection of the piece or costly finishing operations, e.g., multiple sanding operations and the like. Such irregularities in surface smoothness are readily determined by a profile examination of the surface and hence molded articles having a truly smooth surface have been termed "low profile" moldings. Moreover, the polymerizable polyester mixture can be difficult to handle, being sticky or tacky. Thus, when processed into preformed items such as prepregs, sheet molding compounds or extruded rods, the preformed items are difficult to handle and store due to the tacky nature of the materials.
Although attempts have been made to decrease the shrinkage which such polymerizable compositions undergo upon curing, for example by incorporating in such compositions thermoplastic additives such as poly(methyl methacrylate), poly(vinyl chloride), saturated polyesters having hydroxyl terminal groups and the like, such attempts have not been completely successful. Thus they have either failed to significantly reduce cure-shrinkage or they have imparted to the uncured product undesirable physical properties such as tackiness, mold scumming, exudation of the additive, and the like.
Moreover, many such compositions as indicated above are unsatisfactory for use in the recently developed sheet molding compound technique of molding reinforced plastic materials. In this development, the curable mixture is compressed or preformed between sheets of polyolefin, e.g., polyethylene or polypropylene film, and film stripped from the preformed sheets of the curable polyester compositions just prior to molding. In many instances, the polyolefin film cannot be removed cleanly from the polyester resin sheet due to the tacky nature of the latter which may result from the inclusion of the hydroxy terminated saturated polyesters. This is highly undesirable since it complicates the removal of the polyolefin film. Inasmuch as it is conventional to this art, to prepare the sheet and store or ship it before molding, this tacky or sticky conldition is obviously undesirable.
In addition, it is frequently necessary to use relatively large amounts of a high molecular weight additive to control shrinkage adequately which may exude to the surface of the part during molding giving undesireable properties such as poor paint adhesion and mold scumming.
Accordingly, it is an object of this invention to provide polyester compositions suitable for the preparation of the low profile molded articles which in the precured state are not tacky. It is a further object to provide low profile bulk molding compounds and sheet molding compounds which are not tacky and produce molded parts having good gloss.
Other objects and advantages of this invention will be apparent from the following description of the present invention.